There are a number of applications in which substances must be blended in a highly accurate manner. This is particularly true in the paper industry where coloring solutions are prepared by mixing colorants and a diluting agent for treating newsprint to obtain the desired finished color, as the natural paper has a yellowish hue which is unpleasant to the eye.
The traditional method of preparing the paper coloring solution involves mixing highly concentrated color pigments with a diluting agent such as water in a carefully controlled ratio. The mixture is then metered on to the paper. Hue variations in the end product will result if the mixing or the application of the coloring solution to the paper is inconsistent. The dosage of the color pigments is the most delicate step in the mixing operation due to the high ratio of water to color pigments; a small error in the quantity of the pigments results in large color shifts in the final product.
Devices, available on the market, for preparing paper coloring solutions employ variable flow-rate metering pumps for dosing the color pigments. Due to the relatively small quantity of pigments required for preparing a batch of a coloring solution, it has been found that the metering pumps are not sufficiently accurate for delivering the exact quantity of pigments required and, therefore, color variations from one batch to another are almost inevitable.